The present disclosure relates to image sensors. An image sensor is a semiconductor device configured to convert optical images into electrical signals. The image sensor may be classified into two types: a charge coupled device (CCD) type and a complementary metal-oxide-semiconductor (CMOS) type. A CMOS-type image sensor may be referred to as a contact image sensor (CIS). A CIS image sensor may include a plurality of two-dimensionally arranged pixels, each of which includes a photodiode (PD) that converts incident light into an electrical signal. A variety of technologies are being developed to improve performance of the CIS image sensor.